I've got a 1994 BMW 325iC that won't pass smog (for more details, see this thread). I took it to a shop to get both a leak-down and compression test done with some interesting numbers:

Cylinder     1   2   3   4   5   6
Compression 180 210 160 210 205 210
Leak-Down   18% 15% 40% 14% 18% 15%

Obviously it's not holding compression very well across the board, and cylinder 3 is especially leaky. Compression numbers indicate both cylinders 1 and 3 are struggling a bit too.

I was informed by the shop that a quick look into the cylinder revealed quite a bit of carbon buildup, which I can assume is not only on the piston head, but also on the valves and valve seats, which both the shop and I suspect is the primary cause of the leak.

After using some intake manifold cleaning aerosol and dumping nearly two gallons of isopropanol into the tank (shop suggestion, and I trust them), the only truly obvious result is that my idle no longer surges.

Previously, when the engine reached operating temperature, the idle would steadily climb to ~1100RPM over about a half-second, pause for a half-second, then suddenly drop to ~500RPM before repeating the cycle.

Now, it starts up (even when warm) between 600 and 750RPM. After driving for a bit, it will stabilize to ~950RPM, but without surging.

I would like to believe some of the carbon buildup has been removed by my efforts to chemically clean the engine, but I don't know if the elimination of surging is a good indicator of this, which is why I am asking if carbon buildup on the valve seats can cause the engine idle to surge.

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    I cannot give you a true reason for it, but I'd suggest the answer is "no". More than likely there was an issue with the IAC which was causing the surging. Maybe it was sticking which was cleaned out by your chemical therapy. If you did a new leak down you'd probably find the numbers a little better, but not much. I'd suspect you're going to have to pull the head and at the very least, clean the carbon off and lap the valves. At the worst, it could take a valve job. I'm not overly familiar with BMWs, so leaving this as a comment. Apr 11, 2017 at 0:39
  • @Pᴀᴜʟsᴛᴇʀ2 Thanks for the insight. I do believe the ICV (what BMW calls the IAV) got cleaned a bit too, though not enough since it still sticks a bit. Anyway, I plan on a rebuild eventually. For now, I just want to pass smog.
    – Hari
    Apr 11, 2017 at 0:49
  • If the #3 isn't sealing right, you'll never pass smog. When it isn't sealing right, you're blowing all kinds of HC out. I'd give you 10:1 this is the reason you're not passing smog. If you can chem it again you might stand a chance, but more than likely you'll need some kind of remedial action in order to get it to seal ... and the only way to do that is to pull the head. Apr 11, 2017 at 0:58
  • @Pᴀᴜʟsᴛᴇʀ2 Well, the plugs are also pretty fouled, and that could also explain the poor combustion efficiency. Either way, pulling apart the entire block is going to happen at some point.
    – Hari
    Apr 11, 2017 at 0:59
  • Plugs will help, but like I said, if you have a leaking valve, you're going to blow high on HC. Every time the piston comes up on the compression stroke, you're blowing air/fuel out past the valve. Apr 11, 2017 at 1:25


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